This invention relates to the direct reduction of pellets made from an iron oxide-containing material to produce hardened pellets which contain a substantial portion of a metallic iron and are particularly suitable as a charge to iron and steelmaking furnaces.
The demand for hardened, metallized iron pellets has increased significantly in the last decade along the with an increase in the use of electric furnaces for producing steel. Metallized iron pellets having a consistent chemical composition and a reasonably stable price are desired as replacement for scrap steel commonly used as part of the charge in electric steelmaking furnaces. Also, considerably lower capital investment is required to set up a facility for producing steel by the metallized pellets-electric furnace method as compared to the blast furnace-basic oxygen furnace method. This often is a major consideration for smaller companies and developing countries, particularly those countries which do not have a readily available supply of coking coal.
Heretofore, metallized iron pellets have been produced primarily by two different processes, solid-gas and solid-solid. In one type solid-gas process, exemplified by U.S. Pat. Nos. 2,793,109 (Huebler et al) and 3,369,888 (Cruse), iron ore pellets and/or lumps are introduced into a vertical reactor or shaft furnace and are contacted therein with a reducing gas, such as hydrogen and/or carbon monoxide, at an elevated temperature. The reducing gas is diffused into the pellets or lumps and reduces the iron oxides to metallic iron. The reducing gases for this type process usually are supplied by reforming natural or petroleum gases. Consequently, the future of these processes are highly dependent upon the availability of natural or petroleum gases. Also, the metallized pellets often are very porous and highly pyrophoric, requiring additional treatment to make them safe for handling and storage. Furthermore, the reduction reaction takes several hours with attendant operating costs and production rate limitations.
In another type solid-gas process, exemplified by U.S. Pat. No. 3,377,156 (Kalina et al), a bed of particulate iron ore is fluidized with a stream of carbon monoxide-containing gas heated to an elevated temperature ranging up to just below the sintering temperature of the ore. This type process has most of the above-mentioned shortcomings.
In one type of solid-solid process, exemplified by U.S. Pat. No. 2,855,290 (Freemann), 3,029,141 (Sibakin et a l) and 3,386,816 (English), roasted or indurated iron oxide pellets are introduced into the rotary kiln furnace and tumbled in the presence of coke or coal fines, and usually a fluxing agent such as limestone or dolomite, at an elevated temperature. Carbon monoxide, generated by partial combustion of coal or coke, diffuses through the pellets and reduces the iron oxide to metallic iron. An oxidizing gas is required in some cases to generate sufficient carbon monoxide. In this type process, reducability of the pellets depends, among other things, on the temperature of the kiln, porosity of the pellets, particle size of the iron oxide, and reactivity of the coal or coke. Accordingly, the operating temperatures must be closely controlled to a level below the melting temperature of iron oxide in order to keep the pores open and thereby insure diffusion of the reducing gas through the pellets. For some iron oxides, the pellet reduction can take up to as long as 16 hours, require furnaces up to about 410 feet (120 m) in length and consume up to 20 million or more BTU per ton of metallized pellets with no heat recovery.
In the process disclosed in U.S. Pat. No. 3,219,436 (Heitmann et al), moist, green iron oxide pellets dusted or coated with coal or coke fines are introduced directly into a rotary kiln furnace with additional coal or coke and dolomite. The resultant pellets, which are highly porous (low density), are crushed, briquetted and used as a portion of a smelter furnace charge.
In another type solid-solid process, exemplified by U.S. Pat. Nos. 2,806,779 (Case), 3,264,092 (Ban), 3,313,617 (Ban), 3,333,951 (Ban) and 3,938,987 (Ban), carbonaceous fines are mixed with iron oxide fines (in amounts up to that required to reduce 100% of the iron oxides to metallic iron), with or without a fluxing agent such as limestone, to increase the rate of reduction. Pellets formed from the resulting mixture do not have sufficient strength, without further treatment, to withstand the tumbling in a rotary kiln furnace and, therefore, must be reduced on a traveling or circular grate. Consequently, this type process has not been used on a commercial scale.
A particular note should be made of U.S. Pat. No. 3,938,987 (Ban) which teaches that, if a non-agglomerating type carbonaceous material is used in an amount sufficient to reduce all the iron oxide, it interferes with the internal matrix for holding the pellet together. This patent further teaches that this problem can be obviated by reducing the amount of carbonaceous material to 40-80% of that required to reduce the iron oxide to metallic iron. Consequently, the pellets are only partially reduced on a traveling grate. If such partial pellets were used as a charge to an electric steelmaking furnace, substantial amounts of electrical power would be required to complete the reduction.
Canadian Pat. No. 844,529 (Volin et al) discloses incorporating a solid reductant, such as anthracite coal, into mineral ore agglomerates to facilitate the rate of reduction. Green pellets are formed from a moistened mixture of iron ore, the solid reductant and a bonding agent and then hydrothermally hardened at temperatures below the combustion or decomposition of the solid reductant. This patent does not disclose any specific process for directly reducing such pellets.
U.S. Pat. No. 3,770,416 (Goksel) discloses forming green agglomerates from a moistened mixture of iron oxide, such as steel plant waste dust, a sufficient quantity of a carbonaceous material to reduce the iron oxide to metallic iron, and a bonding agent such as calcium oxide, hydrothermally hardening the agglomerates at a temperature below the combustion and decomposition of the carbonaceous material, and then heating the hardened agglomerates at an elevated temperature, such as in a shaft furnace, to reduce a substantial portion of the iron oxide to metallic iron. This patent did not disclose directly reducing such hardened agglomerates in an agitated or tumbling environment.